1. Field of the Invention
The invention relates to a motor or a position indicator (position sensor) comprising an apparatus or method for controlling a brushless electric motor. The term motor is here understood to mean also e.g. a linear motor, whose position is to be detected. Both a rotary motor and a linear motor has as its movable part a rotor.
2. Description of the Related Art
Different types of position sensors are known and are e.g. used for determining the position of motor elements. In most cases there is an associated linear acting principle for a rotary acting principle. The most important classes of such position sensors or indicators are based on the following:
Position sensors with direct contact with the movable element:
a) Mechanical contacts. PA1 a) optoelectric position sensors: light barriers, differential photosensors, optical interference sensors PA1 b) magnetoelectronic (galvanomagnetic) position sensors: Hall effect sensors (based on Hall generators), Wiegand effect sensors, magnetostrictive sensors, magnetoresistive sensors PA1 c) electromagnetic position sensors: resolvers, eddy current sensors, differential transformers PA1 d) electrical position sensors: capacitively acting position sensors, microwave distance measuring equipment. PA1 U.S. Pat. No. 4,495,450 PA1 EP-B 0171635 PA1 DE-OS 27 52 880 PA1 DE-PS 30 13 550 PA1 EP-A 0316077
Position sensors with contactless operating means:
This list only represents the most important classes and only constitutes a small selection of the sensors used in practice for rotary or linear measuring functions. Hereinafter the position determination or detection processes are described to the extent that they are used both in linear or rotary acting electric motors.
Reference is subsequently made to the electrical angular position. The latter is measured in electrical degrees, e.g. 20.degree. el. In the case of a bipolar motor with a number of poles p=1, the electrical degrees of the rotor position correspond to the mechanical degrees. In the case of a four pole motor (p=2), on rotating by 360.degree. mech, the rotor passes through in all p.times.360.degree. el, i.e. 720.degree. el, etc.
The described measuring methods agree within a range of 360.degree. el, i.e. they determine the electrical angular position of the rotor. For example, with a four-pole motor (p=2), the measured electrical angular position 90.degree. el represents a rotor position of 90.degree. el or (90+360).degree. el, which is unimportant for the commutation control.
Besides methods, which carry out a position determination in motors through external sensors (e.g. using Hall effect sensors, light barriers and magnetoresistive sensors), there are methods whose function is to determine the position of a motor by components within the motor. These methods can essentially be subdivided into two classes.
In a first class use is made of the generator characteristics of motor coils and there is a dynamic position determination. As a function of the level or integral of the generated voltage over time, conclusions can be drawn regarding the rotor position.
This class is characterized by different use fields and widely varying embodiments. As an example of the relevant prior art reference is made to the following patent literature:
It is unimportant whether the magnetic field of the rotor is generated by permanent magnets or by electric magnets. To the extent that reference is made here and hereinafter to permanent magnets, it always includes electric magnets, unless indicated otherwise.
In the same way it is possible to functionally interchange rotating and stationary parts of the electric motor, i.e. the invention is suitable for internal rotor-type motors, external rotor-type motors, motors with a planar air gap, motors in which the permanent magnetic rotor rotates and the motor winding is stationary or conversely motors in which the motor winding rotates and is e.g. supplied across slip rings and the permanent magnetic part is stationary and other motor construction types, in the manner illustrated by examples hereinafter.
In a second class the position determination methods are based on the fact that the motors in question are not ideal, i.e. despite constant, distinct electrical quantities, there are normally variable positional and angular dependencies of the mechanical quantities.
Thus, in this case interest is attached to those physical effects, which are based on the position-dependent change of electrical or electromagnetic parameters of the motor and which can be established by checking with electrical means only. Such a check or measurement of interesting parameters can e.g. be performed in that one or more suitable additional measuring coils are fitted at one or more points within the motor. As two-terminal networks, the coils have complex resistors, which are characterized by ohmic resistances and inductance values and said complex resistors are normally also frequency-dependent.
When choosing a suitable fitting point within the motor one or more of the indicated quantities are dependent on the position taken up by the rotor in the case of linear and rotary motors.
Thus, e.g. by the very precise determination of the inductance of the measuring coils, conclusions can be drawn concerning the rotor position, at least with respect to the angular position, as explained hereinbefore. It is disadvantageous that in place of an externally fitted sensor, there must be a sensor in the form of at least one additional measuring coil within the motor.
If there are several such measuring coils, then apart from the complex input resistance and complex output resistance, additionally the coupling ratio of two coils can be used as a measured quantity. There are then several possibilities for measuring the position-dependent quantities in combination. It is also possible to use different frequencies or frequency mixtures.
A method of this type is e.g. described in German patent 26 16 552. It is advantageous that the inductances of the coils can be such that without difficulty higher frequency test signals can be used.
In a technically improved procedure no additional fitting of measurement coils takes place within the motor. Instead of this concomitant use is made of the inductance values of the inductances of the motor, i.e. the strands (phases) or coils of the motor for the measurement process to be performed.
As no additional constructional expenditure apart from the necessary sensor electronics is required in the last-mentioned sensor class, all the members of this class have the advantage of potentially high cost advantages, whilst simultaneously improving the reliability of the motor.
Examples of such arrangements are disclosed by EP-A-251785 and EP-A-171635. In the arrangement according to EP-A-251785 essentially only the additional power stages are required for controlling the motor phases, as well as special components as electronics for controlling the motor. However, the circuit suffers from the disadvantage that a range of 360.degree. el to be measured is split up into two undistinguishable ranges of in each case 180.degree. el. This is due to the fact that symmetrically to the angle of 0.degree. el or 180.degree. el, in the following range of 180.degree. to 360.degree. el there are equally large function values, i.e. it is impossible to know whether a basic value of 0.degree. or 180.degree. el must be added to the calculated rotor position (FIG. 22A). Moreover the precision of the position determination is limited to approximately .+-.60.degree. el.
An arrangement and a method for obviating this problem are described in a work by Watanabe, Isii and Fugi (IECON 87, pp 228-234), whose solution is based on the fact that depending on the pole position of the permanent magnets of the rotor an individual stator coil through the permanent magnetic premagnetization has a current passed through it either more rapidly, or more slowly in the case of a reverse pole position or current application direction.
This is based on the non-linear magnetization characteristic of the participating soft magnetic coil cause or the magnetic circuit and the resulting inductance change of the coil through which current flows. This circuit gives a rotor position determination within a range of 360.degree. el without ambiguity.
However, here again the position determination precision is not adequate and the typically attainable precision is limited to .+-.15.degree. el. However, this precision is sufficient in order to be able to rotate an appropriate motor in planned manner. Above a certain angular speed the voltages induced in the coils are so high, that they can be used for the "dynamic" determination of the commutation times and an acceleration to higher angular speeds is possible. This speed-controlled interchange between dynamic and static position detection is also described in EP-A-251785 and the aforementioned article by Watanabe, Isii and Fuji.
Under suitable conditions, through position-dependent variation of the coil inductances, it is also possible to use the transformer coupling ratio between two coils for determining the position of the rotor. Apart from the amplitude behaviour analysis, a time behaviour analysis can be carried out. The latter method is described in Japanese patent 60-207489 and by Ishii and Watanabe, Mem. Fac. Eng., Osaka City University, vol. 26, pp 57-65, 1985.
For their performance, all the aforementioned methods normally require the use of comprehensive electronic aids.
Methods of this type are in particular used in commutatorless direct current electric motors. As a result of their simple construction and the resulting advantages concerning reliability and efficiency, they are subject to wide-spread use. Essential components of such commutatorless motors are magnets, coils (with or without an iron core), position sensors and control and power electronics. For controlling such motors, use is normally made of galvano-magnetically acting position sensors, e.g. Hall generators and the working magnets of the motor, i.e. its rotor magnets are used as the controlling magnets for said position sensors. However, in such cases the reliability is limited by that of the sensor components, their feedline and by the operating temperature of the sensors and the control magnets.
It is therefore advantageous for improving the reliability of such devices to make no use of external position sensors. In order to carry out a position determination with the components of the motor, it is consequently necessary to concomitantly use as sensor elements belonging to the system, the motor coils--in parallel to their function of driving the motor. A simultaneously obtained advantage is that through the omission of external sensor elements, e.g. the Hall generators, it is possible to save space and consequently smaller structures are obtained. For the operation of the motor without external position sensors, the motor is appropriately alternately brought into the state of a sensor, the state of a motor (actuator), the state of a sensor, etc.
However, it is relatively difficult to detect the position when the motor is stopped. The difficulties more particularly consist of generating sufficiently precise position information within the shortest measurement time and signal processing time from a minimum of measurement information. In a simple case it is sufficient to fix with a reduced precision only the commutation times necessary for changing the current application conditions for the motor. As a function of the motor operation requirements, it may be sufficient to determine the rotor position with an accuracy of approximately .+-.10.degree.. However, in other cases it is e.g. necessary for motor regulating or control purposes to obtain extremely accurate measured values in a very short measurement time, so that a desired speed correction or position check is obtained. This sets increased demands on the power of the associated electronics and software used. Another difficulty in the known methods is to exclude type-typical phase errors of .+-.180.degree. el in the case of a rotor position determination. Such errors mean that if the rotor is further rotated by 180.degree., 360.degree. el, etc., the typical position information remains largely unchanged compared with the original position.